Air springs, both rolling lobe and bellow, are used in vehicle suspension systems, most often in large commercial trucks. In some instances air springs are used to provide suspension between the truck frame and the wheel axles. In other instances, air springs are placed between the frame and the passenger cab. Traditionally, air springs are fed from a source of compressed air to maintain a predetermined internal pressure. The air springs can be in communication with valves to regulate the pressure and volume or height of the springs. The desired pressure of the air springs is affected by the load on the suspended element (in a truck, the frame or cab). For example, the proper ride height for both the frame and the cab can be obtained by increasing the air spring pressure to accommodate the load on the frame or cab. These air springs have produced an improved ride quality when compared to traditional springs due to a reduced spring rate and reduced natural frequency.
FIG. 1 shows a prior art air spring system 2. The system comprises an air spring 4 and shock absorber 8 disposed between a cab 10 and truck frame 20. The system further comprises a level sensitive air valve 12. The air valve 12 connects to a source of pressurized air 30 by a first conduit 14 and to the air spring 4 by a second conduit 16. Based on the level of the cab 10 relative to the frame 20 (as monitored by a level sensor), the valve 12 will transfer air into the air spring 4, vent air from the air spring 4 to the environment, or seal the air spring to maintain the spacing. These valves 12 can be mechanically adjusted to determine the proper distance between the cab 10 and the frame 20 as is known to those of ordinary skill in the art. Typically, a truck cab will have two air springs 100, one at each corner of the cab 10, as seen in FIG. 2.
Current air spring systems can be less effective when used in rough or off-road environments. During off-road travel, jouncing of the truck can lead to repeated oscillation of the cab 10 relative to the frame 20. This oscillation can continue for long periods, greatly affecting the comfort of the driver. The frequency of input forces from these rough road surfaces can vary greatly. As a result, there is an increased probability for the input frequency to equal one of the natural resonance or harmonic frequencies of the truck or cab structure. This frequency matching or overlapping can multiply greatly the effects felt by the driver and can damage the truck or cab.
Further, prior art air spring systems can allow an uncomfortable roll of the cab when the truck makes a turn. The forces while turning will result in the cab having a desire to lean away from the center of the turn. With the air springs of the prior art, the cab level sensor is located at a center of the cab between the air springs at each corner and may not sense cab roll. Accordingly, the prior art air springs will not change to counter a cab roll event.
As seen in FIG. 1, shock absorbers 8 have been added to prior art air springs to help dampen oscillation. However, resonance occurs at different frequencies for different load conditions of the vehicle. Therefore the single damping force provided by the shock may only provide assistance under certain conditions.
There remains a need for an air spring system that is able to minimize repeating oscillations during a jouncing event. There is a need for an air spring system that provides oscillation damping over a range of road input conditions. There is a need for an air spring system that is able to handle the oscillations during off-road use while maintaining comfort and the proper ride height during highway use. There is also a need for air springs having independent components to reduce cab roll while the truck is turning.